nodejs: Fix build by pinning to icu68

Node 12/14/15 don't build against icu69, failing with:

```
../deps/v8/src/objects/js-list-format.cc:172:55: error: 'createInstance' is a private member of 'icu_69::ListFormatter'
```

pkgs/development/web/nodejs/v12.nix

@@ -1,8 +1,8 @@
-{ callPackage, openssl, icu, python2, lib, stdenv, enableNpm ? true }:
+{ callPackage, icu68, python2, lib, stdenv, enableNpm ? true }:
 
 let
   buildNodejs = callPackage ./nodejs.nix {
-    inherit openssl icu;
+    icu = icu68;
     python = python2;
   };
 in

pkgs/development/web/nodejs/v14.nix

@@ -1,8 +1,8 @@
-{ callPackage, openssl, python3, lib, stdenv, enableNpm ? true }:
+{ callPackage, icu68, python3, lib, stdenv, enableNpm ? true }:
 
 let
   buildNodejs = callPackage ./nodejs.nix {
-    inherit openssl;
+    icu = icu68;
     python = python3;
   };
 in

pkgs/development/web/nodejs/v15.nix

@@ -1,8 +1,8 @@
-{ callPackage, openssl, python3, enableNpm ? true }:
+{ callPackage, icu68, python3, enableNpm ? true }:
 
 let
   buildNodejs = callPackage ./nodejs.nix {
-    inherit openssl;
+    icu = icu68;
     python = python3;
   };
 in